1. Field of the Invention
The present invention relates to a rotary-anode type X-ray tube, more particularly to improvement of the rotary structure for rotating an anode target.
2. Description of the Related Art
As is known, in a rotary-anode X-ray tube, a disk-like anode target is supported by a rotary structure and a stationary structure. The rotary structure is connected to the stationary structure by a bearing portion. An electron beam emitted from a cathode is applied to the anode target while the anode target is rotated at a high speed by energizing an electromagnetic coil arranged outside a vacuum envelope, thus irradiating X rays. The bearing portion is constituted by a rolling bearing such as a ball bearing or a hydrodynamic pressure type sliding bearing which has bearing surfaces with spiral groove on the bearing surface and uses a metal lubricant consisting of, for example, gallium (Ga) or gallium-indium (Ga-In-Sn) alloy, which is liquefied during an operation. The rotary-anode type X-ray tubes using the sliding bearing are disclosed in, for example, Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-97536, 60-113817, S60-117531, S61-2914, and S62-287555.
The rotor assembly includes a rotary shaft supporting the anode target and made of a metal having a high melting point, a cylindrical inner core functioning as a rotor of a motor, fixed to the rotary shaft, and made of a ferromagnetic material such as iron, and an outer cylinder made of a material having a high electric conductivity such as copper. The inner core is fitted in the outer cylinder and brazed to the outer cylinder A stationary shaft is inserted into the rotary cylinder. A bearing connects the stationary shaft and the rotary cylinder. In the rotor assembly, the rotor is rotated at a high speed, according the theory of an induction motor, when a rotating magnetic field is applied to the rotor from a stator outside the tube.
When the sliding bearing is filled with liquid metal used as lubricant, there is the advantage that the bearing surface is hardly abraded and rotational noises are hardly produced. However, in the X-ray tube, the following problems occur. While the X-ray tube operates, the rotor cylinder supporting the anode target is heated due to the heat transmitted from the target. Therefore, the hydrodynamic pressure type sliding bearing is also heated. Thus, when the bearing is filled with liquid metal, various problems may occur. For example, the rotational performance of the bearing is degraded due to a change of the bearing gap distance, and the liquid metal reacts on the bearing material because it is active. Therefore, as disclosed in the above-identified patent applications, the above problem can be solved by forcibly cooling the sliding bearing with circulating coolant. To this end, the X-ray tube apparatus must have coolant circulating paths and a coolant circulation system. Thus, the system can be use only for a special equipment because it is not compatible with the conventional X-ray tube.